1. Field of the Invention
The present invention relate to oxide superconductors and a process for preparing them. More precisely, the present invention provides oxide superconductors having a structure completely different from those of conventional superconductors and a process for preparing them, wherein an oxide superconductor layer is formed on an oxidized metal layer by heating a solidified melt comprising oxide superconductor precursor to a temperature above the partial fusion temperature so that the solidified melt is effused through a metal tube.
The superconductors of the present invention are suitable for conductors such as hollow coils which may be used as a path for cooling medium.
2. Prior Art
Oxide superconducting materials such as those of the bismuth system have a markedly higher critical temperature (Tc) than metal superconductors such as those of the niobium-titanium system and hence they are economical materials since relatively cheap liquid nitrogen can be used as cooling medium therefor in place of expensive liquid helium. Therefore, practical use of oxide superconductors has been anticipated in various fields including electronics, power transmission and strong magnetic field generation.
Further, their critical current density (Jc) is today nearing a practical level. In order to realize practical use of oxide superconductors on a large scale, it is indispensable to establish techniques for making them into elongated articles, i.e., making them into wires. However, since oxide superconductors are brittle and difficult to process by cutting, it is difficult to make them into wires or coils.
Hitherto, the metal sheath (Ag sheath) method and the organic acid salt method have been known as effective methods for making superconducting wire materials. In the former method, a superconducting wire is manufactured by subjecting a metal tube charged with material powder to a reduction process such as a wire drawing process and rolling process and sintering the tube to grow the material powder into a superconductor. Since the superconductor is covered by the metal tube, it is very easily processed and hence this method is suitable for the production of a wire of long length. In particular, in the case of the bismuth system superconductors, i.e., those of a molar ratio of Bi:Sr:Ca:Cu=2:2:2:3 (hereinafter referred to as "(2 2 2 3) phase"), a wire having a high Jc can be obtained since the crystal faces are oriented in a single direction by the above-described reduction process. In the latter method, for example, a Bi system oxide superconductor can be produced by dissolving organic metal compounds such as organic acid metal salts and metal alkoxides containing elements which constitute the superconductor, in a predetermined ratio in an organic solvent, applying the solution on a substrate and subjecting it to heat treatment.
However, in order to produce a conductor having an internal hollow which may used as a path for cooling medium by these methods, wire materials including tapes have to be combined with cooling pipes. This makes the production of such composite materials complicated and difficult.
Thus, it is difficult to produce an oxide superconductor having an internal defining a path for liquid nitrogen as is required for application to power equipment, and hence it remains delayed in practical application.